the long-term objective of this grant is to elucidate the relationships between structure, electronic properties, and dynamics of tetrapyrroles. These relationships are crucial for the functioning of porphyrins and related molecules in numerous biological systems but are difficult to explore in vivo. However, the functionally-important properties of tetrapyrroles and the diagnostic spectroscopic characteristics of the associated transient states can be systematically examined through time- resolved and steady state spectroscopic studies of simple tetrapyrrole systems in vitro. This proposal focuses on three areas of fundamental importance to the study of the heme proteins and the bacterial photosynthetic reaction center. First, the composition of the electronic states and the associated optical properties and relaxation dynamics of supermolecules consisting of two tetra-pyrroles held in van der Waals contact will be explored. The special properties of strongly-coupled porphyrin dimers such as the bacteriochlorophyll dimer of the reaction center will be investigated using a series of symmetric and asymmetric porphyrin sandwich complexes that differ incrementally in the distance between the macrocycles. Second, the manifestations of excess vibrational energy and the rates and mechanisms of vibrational relaxation in porphyrinic systems will be addressed through ultrafast and steady state optical studies of a series of tetrapyrrole complexes in the condensed and gas phase. It is becoming increasingly clear that vibrational excitation and dynamics widely affect time-resolved spectroscopic measurements of heme proteins and the reaction center and also may be functionally important for the latter. Finally, the proposed research will address the excited state optical properties and relaxation dynamics of tetrapyrrole pi-cation and pi-anion radicals. This is an essentially unexplored area that warrants serious attention considering that porphyrin ionic intermediates are intimate to the function of numerous tetrapyrrole-protein complexes. These important issues will be addressed using femtosecond and slower transient absorption spectroscopy and steady state optical techniques. Collectively, the work will help elucidate the properties of tetrapyrroles that underpin their diverse biological functions.